For this one You have to create a Job
Go through the below link for How to habdling Josbs in sqlserver
Jobs in sql server[
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You have to maintain last 3 months data.., Don't TRUNCATE the tables(as Wasif) said, this leads to a Data Loss.
Execute DELETE Quires in Job that delete only before 3 months data..,
At time of Job execution You have total 6 months data(if you are executing it for every 3 months)